In recent years, as semiconductor devices are increasingly more integrated, circuit wires are made thinner, and the dimensions of integrated semiconductor devices are made smaller and smaller. This leads to the need for a process of removing a coating formed on the surface of a wafer to planarize the surface, and as an approach to this planarizing method, the wafer is polished by a chemical mechanical polishing (CMP) apparatus. The chemical mechanical polishing apparatus comprises a polishing member such as polishing cloth, pad and the like; and a holding member such as a top ring, a chuck and the like for holding an object under polish (i.e., an object being polished). The apparatus presses a surface to be polished against the polishing member, and relatively moves them while supplying a polishing assistant such as an abrasive liquid, a chemical liquid, a slurry, pure water or the like, thereby polishing the surface of the object under polish into a flat and mirror-surface state.
In this type of chemical mechanical polishing apparatus, the polishing member mainly has a discoidal or an annular shape, and polishing apparatuses can be classified into a large-diameter polishing member rotation type, a small-diameter polishing member rotation scanning type, and the like depending on the relationship of magnitude between the polishing member and an object under polish. A polishing apparatus classified as the large-diameter polishing member rotation type rotates an object under polish which is held by a top ring with a surface to be polished being oriented downward, i.e., in a face-down arrangement, and presses the object under polish against a turn table provided with a polishing member larger than the object so as to polish the object. The polishing member is generally rotated by the turn table. On the other hand, a polishing apparatus classified as the small-diameter polishing member rotation scanning type rotates an object under polish which is held by a chuck with a surface to be polished being oriented upward, i.e., in a face-up arrangement, and presses a polishing member smaller than the object against the surface to be polished, while rotating and scanning the polishing member, to polish the object.
In either of the foregoing polishing apparatuses, part of the polishing member temporarily or always extends off the object under polish. This extending polishing member causes an excessive polishing pressure to be applied around the edge of the object under polish, resulting in a degraded flatness around the edge of the object under polish. For this reason, the yield rate of semiconductor devices exacerbates in a wafer formed with the semiconductor devices. This is because more semiconductor devices exist toward the outer periphery of the wafer. Therefore, one challenge imposed to the polishing apparatuses is to extend a region of high flatness as close as possible to the edge, such that the polishing apparatuses can sufficiently support edge exclusion defined by semiconductor device manufacturers and the like.
It is known that the aforementioned excessive polishing pressure is produced because part of the polishing member extending off an object under polish and remaining open is abruptly oppressed by a pressure exerted on the object under polish by the motion of the polishing member relative to the object under polish, i.e., a pressing pressure produced when the polishing member and object under polish are moved while they are kept in contact with each other. Such a phenomenon is called “rebound.” The rebound also occurs when a polishing member pressed onto an object under polish extends off the object under polish and is released from the pressing pressure.
In addition to the rebound, the small-diameter polishing member rotation scanning type is generally configured to allow the polishing member to swing together with a mechanism for holding the polishing member, so that the polishing member extending off an object under polish causes the polishing member to incline over the entire surface, causing the pressure to further increase on the edge of the object under polish.
For preventing such an excessive polishing pressure from being applied around the edge of an object under polish, large-diameter polishing member rotation type polishing apparatuses generally have a retainer ring to surround the object under polish at the holding member, such as a top ring for holding the object under polish, such that the polishing member around the object under polish is pressed by the retainer ring to prevent the rebound. This is intended to control the influence of the rebound by a pressure with which the polishing member is pressed against the retainer ring. Therefore, the large-diameter polishing member rotation type polishing apparatus is generally operated after a dummy wafer is previously polished on a trial basis to find, from the result, a pressure condition for the retainer ring under which the rebound exerts a smaller influence and a region of high flatness can be extended as close as possible to the edge, and this pressure is set as a retainer ring pressure.
Also, a method of further reducing the influence of the rebound includes controlling a contact pressure in an edge zone of a wafer using a profile control type top ring for a holding member. This profile control type top ring is configured such that a pressure (pressing pressure) with which a wafer is pressed can be set for each of the areas (pressing section) concentrically partitioned on an object under polish. It is therefore possible to control a pressing pressure for a pressing section (associated with an edge area) which serves the edge area of the wafer independently of other areas. When a pressing pressure in the edge area is made lower than those in other areas, it is possible to limit an excessive pressure due to the rebound.
Therefore, in a large-diameter polishing member rotation type polishing apparatus provided with a profile control type top ring, a dummy wafer is previously polished on a trial basis, as is done for finding a pressure condition for the retainer ring, to find, from the result, a pressing pressure condition for an edge area under which the rebound exerts smaller influence, and a region of high flatness can be extended as close as possible to the edge, and this pressing pressure is set as an edge area pressure before the apparatus is operated. It should be noted that since both the retainer ring pressure and edge area pressure affect the flatness of the wafer edge, a pressure condition must be found for both pressures, rather than finding respective pressure conditions independent of each other, in order to find a more preferable pressure condition.
On the other hand, in regard to the small-diameter polishing member rotation scanning type polishing apparatuses, Laid-open Japanese Patent Application No. 2001−244222 (Patent Document 1), Laid-open Japanese Patent Application No. 2002−75935 (Patent Document 2), Laid-open Japanese Patent Application No. 2002−134448 (Patent Document 3), and Laid-open Japanese Patent Application No. 2003−229388 (Patent Document 4) disclose apparatuses, each of which comprises a supporter for supporting a polishing member which extends off an object under polish to prevent the rebound and inclination of the polishing member and can therefore reduce the edge exclusion. The supporters disclosed in Patent Documents 1−4 perform an action corresponding to the retainer ring in the large-diameter polishing member rotation type polishing apparatus. In the small-diameter polishing member rotation scanning type polishing apparatus, the rebound and inclination of the polishing member can be controlled by the height of a supporting surface of the supporter, for example, a relative height from the top surface of a chuck. Therefore, such a polishing apparatus is operated after a dummy wafer is previously polished on a trial basis to find, from the result, a condition for the height of the supporting surface under which the rebound and inclination of the polishing member exert smaller influences, and a region of high flatness is extended closer to the edge of wafer, and this height is set as the height of the supporting surface.
Thus, in the small-diameter polishing member rotation scanning type polishing apparatus, when an object under polish has a varying thickness, the height of the supporting surface must be adjusted in accordance with the thickness of the object under polish in order to extend a region of high flatness as close as possible to the edge, as described in Patent Document 4. However, in the large-diameter polishing member rotation type polishing apparatus, when the retainer ring is used, variations in the thickness of an object under polish hardly cause a problem because a retainer ring pressure can be controlled.
An edge zone of a bare wafer includes a portion which is inferior in flatness and departs from an ideal shape, as compared with the center of the wafer. Such a shape in the edge zone of the wafer is called “wafer edge roll off” (hereinafter simply called the “roll off”). Not only the bare wafer but also an oxide film wafer polished by a CMP apparatus, for example, when STI (Shallow Trench Isolation) is formed to separate devices presents the roll off derived from the roll off of a bare wafer before the CMP-based polishing. The shape of roll off varies from one wafer to another. Even with the same thickness, the roll off differs. Also, even in a single wafer, there are generally variations in the circumferential direction.
In double-side polished 300-mm wafer used for recent semiconductor integrated circuits, a deviation from a flat surface due to the roll off at a position of 1 mm inwardly from the edge of the wafer is not more than approximately 1 μm at most. However, Akira Hukuda, Hirokuni Hiyama, Manabu Tsujimura, Tetsuo Hukuda, “Influence of Wafer Edge Roll-off on Polishing Profile of CMP,” 2004 The Japan Society for Precision Engineering, Autumn Academic Lecture Meeting Collected Papers, p. 497−498. (Non-Patent Document 1), which was published by the present inventors and others, clarified that the roll off affects a polishing profile up to approximately 5 mm inwardly from the edge of a wafer. Here, a current edge exclusion is prevalently 3 mm, and will be 2 mm in the near future with certainty, so that it is understood that the influence of the roll off reaches into the edge exclusion.
As described above, the polishing method according to the prior art involves previously polishing a dummy wafer to find, for example, a pressure to be applied to a polishing pad by a retainer ring in a large-diameter polishing member rotation type polishing apparatus, or, for example, the height of a supporting surface of a supporting member in a small-diameter polishing member rotation scanning type polishing apparatus, setting the pressure or height as a retainer ring pressure or the height of the supporting surface, and operating the apparatus. However, in such a polishing method, if the roll off varies from one wafer to another, the polishing profile also varies, resulting in the inability to extend a flat region to the vicinity of the edge. In other words, a problem arises in the inability to sufficiently support a set edge exclusion. Also, when the roll off varies in the circumferential direction, the polishing profile varies in the circumferential direction, leading to a problem of the inability to extend the flat region to the vicinity of the edge.
For representing the shape of roll off, one can define that a roll off quantity (ROQ) is a set of the distances between several points on a surface to be polished of an object under polish and a reference line which passes a reference point and is substantially parallel with the surface to be polished of the object under polish. FIG. 27 schematically illustrates a cross section, which passes the center of a wafer, used as an object under polish, by emphasizing the value of ROQ and changing the aspect ratio. When the radial direction of the wafer is only taken into consideration, the roll off quantity is a set of the distances between several points on a line indicative of a surface to be polished, appearing on the cross section passing the center of wafer, and the reference line, for example, when the reference point is set above the surface to be polished. For example, when the distance from the center of the wafer is designated by r, the roll off quantity at r is ROQ (r), as shown in FIG. 27. While the radial direction alone is taken into consideration in FIG. 27, the roll off quantity also changes in the circumferential direction, so that it is uniquely determined by the position on the surface to be polished, and the roll off quantity can be represented by ROQ (r,θ), when the coordinates of the surface to be polished is taken on polar coordinates (r,θ) which have the origin at the center of the surface to be polished.
In the foregoing description, the reference point is set above the surface to be polished, but the reference point may be set on the surface to be polished or below the surface to be polished. Also, while polar coordinates are employed in the foregoing description, the coordinate system may be orthogonal coordinates. Further, the reference line may be a straight line substantially parallel with the overall surface to be polished, or may be a straight line substantially parallel with part of the surface to be polished, for example, a range of radius r1 to r2 within the surface to be polished (where r1<r2).
Further, when the value of ROQ is measured not only in the radial direction but also in the circumferential direction, a reference plane may be employed instead of the reference line when the surface to be polished is regarded as two-dimensional. In this event, the reference plane may be a plane substantially parallel with the overall surface to be polished, or a plane substantially parallel with part of the surface to be polished. Also, in measuring and using the roll off quantity, the distance between one point on the surface to be polished and the reference line (or reference plane) may be measured and used, instead of a set of the distances between a plurality of points on the surface to be polished and the reference line (or reference plane).
M. Kimura, Y. Saito, H. Daio, K. Yakushiji, A New Method for the Precise Measurement of Water Roll off of Silicon Polished Wafer, Jpn. J. Appl. Phys., Vol. 38 (1999) Pt. 1, No. 1A, p 38-p 39 (Non-Patent Document 2) shows an example in which a straight line substantially parallel with a region of a wafer in a range of 3 mm to 6 mm from the outer edge of the wafer as part of a surface to be polished is chosen to be a reference line, a position at 1 mm from the outer edge of the wafer is set as a point on the surface to be polished, and the distance between the position and the reference line is measured. This value is called ROA (Roll Off Amount).
The inventors have employed a numerical analysis approach and found when ΔROQ, later described, is only changed that under the same polishing conditions, including a pressure acting between a polishing member and a wafer, a pressure acting between the polishing member and a retainer ring, and the like, a maximum polishing rate and a minimum polishing rate change inside the edge exclusion as ΔROQ is different. Assume hereinafter that the maximum polishing rate and minimum polishing rate indicate values inside the edge exclusion unless otherwise noted. Here, ΔROQ means a value calculated by ΔROQ=ROQ1−ROQ0, where ROQ0 is the roll off quantity at the center of a wafer, and ROQ1 is the roll off quantity at a location 1 mm from the wafer edge, for example, on a surface to be polished of a wafer. The value of ROQ1 may be an average of ROQ's at respective points in the circumferential direction of the wafer W or the value only at a single point used as a representative value. It has been recognized, on the other hand, that polishing can be carried out with a practically sufficient flatness as long as the maximum polishing rate and minimum polishing rate fall within an appropriate range.
Conventionally, when the roll off varies among wafers, the polishing profile also varies, occasionally resulting in a situation in which the flatness exacerbates, but it has been revealed from the foregoing finding that this is caused by variations in roll off which force the maximum polishing rate or minimum polishing rate or both to extend off an appropriate range. For reference, the polishing rate means the rate at which a surface to be polished is polished, and generally indicated by the velocity. For example, its dimension can be represented by [length]/[time]. In the present invention, this dimension is further divided by pressure to derive the rate per unit pressure which is used as the polishing rate. Also, in this specification, the polishing profile refers to the shape of a distribution of the polishing rate within the wafer surface.
The inventors diligently investigated, using numerical analyses, a means which can control the polishing rate to an appropriate value to find a preferred polishing profile, and reached to attain the following findings. As a first finding, it was found that when wafers having the same ΔROQ were polished while the retainer ring pressure alone was changed under the same polishing conditions including the pressure between the polishing member and wafer and the like, the maximum polishing rate and minimum polishing rate changed in accordance with the retainer ring pressure (see FIG. 3 in regard to this finding). Drawing inspiration from this fact, the inventors found that the object under polish can be polished with a practically sufficient flatness by adjusting the retainer ring pressure in accordance with ΔROQ of a wafer such that the maximum polishing rate and minimum polishing rate fall within an appropriately set range.
A second finding is that when a numerical analysis was made on wafers having the same ΔROQ while the height of the supporting surface alone was changed with the rest of polishing conditions remaining the same, the maximum polishing rate and minimum polishing rate also changed in this case. Drawing inspiration from this fact, the inventors ended up to think that the object under polish can also be polished with a practically sufficient flatness by adjusting the height of the supporting surface in accordance with ΔROQ of the wafer such that the maximum polishing rate and minimum polishing rate fall within an appropriately set range (see FIG. 8 in regard to this finding).
As a third finding, in a large-diameter polishing member rotation type polishing member provided with a profile control type top ring as a holding member, it was found that when a numerical analysis was made on wafers having the same ΔROQ while the pressing pressure alone was changed for an edge area with the rest of polishing conditions remaining the same, the maximum polishing rate and minimum polishing rate also changed in this case. Drawing inspiration from this fact, the inventors ended up to think that the object under polish can also be polished with a practically sufficient flatness by adjusting the pressing pressure for the edge area in accordance with ΔROQ of the wafer such that the maximum polishing rate and minimum polishing rate fall within an appropriately set range.
As a fourth finding, in a large-diameter polishing member rotation type polishing member provided with a profile control type top ring, the inventors ended up to think that the object under polish can also be polished with a practically sufficient flatness by adjusting both the pressing pressure for the edge area and the retainer ring pressure in accordance with ΔROQ of the wafer such that the maximum polishing rate and minimum polishing rate fall within an appropriately set range. Also, when a numerical analysis was made while changing the modulus of elasticity and the thickness of a polishing member, it was found that the influence of the retainer ring pressure and the pressing pressure for the edge area on the polishing rate changes depending on the modulus of elasticity and the thickness of the polishing member. As a result of diligently studying from the foregoing, it was found that there are respective ranges for the modulus of elasticity and for the thickness of the polishing member suitable for polishing a wafer with a practically sufficient flatness by adjusting the retainer pressure and the pressing pressure for the edge area in accordance with the roll off of the wafer.
The present invention has been made in view of the foregoing challenge and findings, and it is an object of the invention to provide a polishing apparatus and a polishing method which are capable of polishing an object under polish with a high yield rate even if the object under polish presents a roll off. Further, it is an object of the present invention to provide a semiconductor device manufacturing method which is capable of manufacturing semiconductor devices at a low cost, and to provide low-cost semiconductor devices.